Press forming of a metal sheet is used in a very broad range of fields from the manufacturing of industrial products such as automobiles, household appliances, and office equipment to the manufacturing of daily necessaries such as beverage cans, sink cabinets, and bathtubs, and occupies an important place in the field of plastic working. In recent years accordingly, in order to reduce the weight of an automobile for example, it has been expected to apply press forming to a material such as a high-tensile steel sheet, an aluminum alloy sheet, or the like. However, such a metal sheet has poorer formability than a cold-rolled steel sheet. Consequently, a technology for improving formability of a high-tensile steel sheet or the like and facilitating the press forming thereof has been demanded.
In the case of press forming, with the aim of preventing the damages caused by poor lubricity on the surfaces of a die and a metal sheet, processing of improving lubricity and workability by coating a metal sheet with pressforming lubricant oil has heretofore been applied. However, a problem in the use of pressforming lubricant oil is that the work environment deteriorates because of the scatter of the pressforming lubricant oil. In addition, in recent years, the influence of a chlorine-based organic solvent or the like used for the cleaning of pressforming lubricant oil on environment has also been pointed out.
In view of the above situation, as a technology which allows excellent formability to be expected, and moreover, the deterioration of work environment and the increase of environmental load caused by the scatter of pressforming lubricant oil and the evaporation of a cleaner to be suppressed, known is a method of forming a resin layer excellent in lubricity beforehand on the surface of a metal sheet. The technology is a means whereby a material manufacturer as the supplier of a metal sheet forms a resin layer beforehand (pre-coat) on the surface of the metal sheet and thereby, when a customer processes the metal sheet, the customer can press-form the metal sheet without the use of pressforming lubricant oil or the like. Such resin-coated metal sheets are roughly classified into the following two types.
One type is a non-stripping type resin-coated metal sheet which is designed so as to be able to use a resin coating layer as the final coated film as it is in order to simplify the process of press forming. However, problems of the metal sheet are that the metal sheet has poor electrical conductivity since the resin coating layer remains on the surface of the metal sheet even after the press forming, and that makes it difficult to apply chemical conversion treatment, electrodeposition coating, welding, and others. In addition, it is extremely difficult technically to impose properties, such as chemical resistance, corrosion resistance, and scratch resistance, required for the final coated film and at the same time stamping performance on the resin layer.
The other type is a resin-coated metal sheet which is designed, on the premise that the final coated film is formed in another process, so as to form a stripping type resin layer excellent in lubricity beforehand and then remove the resin layer by alkaline cleaning or the like after press forming before the forming of the final coated film. With the metal sheet, the resin layer can be removed by using an alkaline degreasing process conventionally incorporated into a coating line for metal sheets and hence extra cost can be avoided and the metal sheet can conveniently be used.
As such a stripping type resin-coated metal sheet, JP-A No. 38539/2000 discloses: a composition (paint) of an alkali stripping type, wherein the composition is a water-soluble copolymer having methacrylate ester or the like as the monomer, and the glass-transition temperature and others of the methacrylate ester are stipulated; and a steel sheet on which a coated film is formed with the paint. It is said that the object of the technology is to improve the rust resistance and stamping performance of a steel sheet.
Further, JP-A No. 502089/1996 describes a technology related to a metal which has been subjected to lubrication treatment with a lubricant containing partial ester of dihydroxy compounds or the like such as ethylene glycol monolaurate as an essential ingredient. It is said that the film has lubricity suitable for press forming operation and can easily be removed with a water-based alkali cleaner.
Furthermore, JP-A No. 172776/2001 discloses an alkali-soluble type lubricant-coated stainless steel sheet on which a film containing polyolefin wax or the like is formed as a lubricity-imparting agent, and JP-A No. 371332/2002 describes an aluminum alloy sheet having a similar film. Those films have alkali film removability and certain formability.
Further, JP-A No. 323286/1996 discloses an aluminum alloy sheet having a film of polyethylene oxide whose average molecular weight is 50,000 to 5,000,000, namely polyethylene glycol of relatively high molecular weights, on the surface thereof. In addition, International Publication WO 95/18202 describes: a lubricant containing polyalkylene oxide, such as polyethylene oxide, and higher fatty acid salt; and an aluminum or aluminum alloy sheet coated with the lubricant.
As stated above, resin-coated metal sheets aimed at having both the lubricity and film removability of resin layers have heretofore been known. However, the resin layer used for such a conventional resin-coated metal sheet has not had such sufficient lubricity as to be applicable to an aluminum alloy sheet or the like which is hardly press-formed and resultantly the formability of the metal sheet has not been sufficient in some cases.
For example, the resin layer disclosed in JP-A No. 38539/2000 has poor lubricity and hence the stamping performance of a steel sheet coated with the resin layer is insufficient. Further, in the cases of the resin layers disclosed in JP-A Nos. 172776/2001 and 371332/2002 too, it is estimated that formability is still insufficient when they are applied to a material which is hardly processed.
In the meantime, JP-A No. 323286/1996 and International Publication WO 95/18202 describe an aluminum sheet and the like having a resin layer containing polyethylene oxide and the formability of the resin layer is experimentally verified. However, in consideration of the properties required for an aluminum sheet and the like in recent years, it is necessary to further improve the formability.
Further, when conventional resin-coated metal sheets are piled into a stack and stored, the resin layers have sometimes stuck to each other (hereinafter, this phenomenon is called “blocking”) particularly in the case where a large number of sheets are piled, in a tropical region, or in summer season. Once such blocking occurs, not only it becomes difficult to take out the metal sheets one by one at the time of use and operability deteriorates but also the coating weight of the resin is uneven due to the exfoliation or the like of the resin layer and stable formability cannot be obtained.
For example, the resin layer disclosed in JP-A No. 502089/1996 is mainly composed of low molecular weight compounds having a low melting point, and hence is likely to stick to another resin layer, and is also inferior in blocking resistance. Moreover, another problem is that, when the resin layer is exposed to a high pressure at the time of forming, the resin layer is likely to be pushed out of the working plane and thus is inferior in formability. In addition, film-forming capability when the resin layer is formed on a metal sheet is also insufficient.